Are mining systems suitable for use as a render farm?
Judging from Blenchmark, yes. I realize that it has been closed down for a while so its results should be taken with a grain of salt, but I don't think they're completely irrelevant.



The mobo you posted should work fine if you don't intend on using anything too powerful/modern. The PCI-E 2.0 slots may bottleneck the GPUs connected to it if you connect them to GPUs that have been designed with higher bandwidth PCI-e slots in mind.



As far as the smaller PCIe slots and connecting cables go, I don't think you'll get 16 GPUs hooked up to that. What is most likely going on is that multiple slots are connected to a single GPU using those cables. I'd also be wary of potential hardware/driver issues when doing so. However, I'm not familiar with that subject, so keep that in mind.